The goal of conformal radiation therapy is to deliver a curative radiation dose to the tumor while sparing nearby healthy tissue. In intensity modulated proton therapy (IMPT), this is accomplished by delivering many proton pencil beams of different sizes and ranges throughout the target volume, and in intensity modulated [photon] radiation therapy (IMRT) and related modalities, this is accomplished by shaping the radiation beam using a multi-leaf collimator. The resulting dose distributions can be highly complex, and must be verified by detailed 3D measurements. Current 2D dose measuring devices may miss vital differences between the planned and delivered doses because they cannot measure the full 3D dose distribution. At this time there are no suitable detectors available for accurate, high resolution, and efficient 3D dose measurement for verification of complex photon and proton dose distributions. Our long-term goal is to reduce radiation treatment errors and improve dose verification accuracy by developing a new fast, reusable 3D detector for patient treatment verification. The objective of the proposed research is to develop a 3D detector based on the measurement of light emission from a large volume of scintillator, and use this detector to obtain quality assurance measurements for IMPT and IMRT treatments. On the basis of our preliminary work, we hypothesize that a volumetric scintillation detector can measure 3D dose distributions in real time with an accuracy of 3% or better. The rationale for this project is that it will enable complete on-line high-resolution 3D dose measurements as a part of routine QA for each patient, while decreasing considerably the time required for treatment verification. To reach this goal, we aim to: a) develop instrumentation and reconstruction techniques to measure 3D light distributions in a volumetric scintillator detector, b) develop quenching correction methods for scintillation dosimetry of proton beams, and c) validate the 3D scintillation detector for radiotherapy treatment verification. The proposed research is significant because it will produce a fully three-dimensional dosimetry system for radiotherapy quality assurance. The system will be efficient and cost effective and will improve confidence in dose distributions delivered to patients. It will also decrease the time required for verification measurements, removing a major workflow bottleneck and allowing more patients to benefit from IMPT and other complex radiotherapy modalities. This is particularly important with the rapid increase in the number of proton therapy centers nationwide and worldwide. The proposed project is highly innovative in the sense that it will lead to a first-of-its-kind dosimetry system capable of instantaneously measuring complex 3D dose distributions. In addition, we expect this detector to be valuable for 3D dosimetry of other treatment modalities, including stereotactic body radiation therapy, passive scattering proton therapy, and even high-dose rate brachytherapy.